Method of and apparatus for coating a substrate with a coating material having an even thickness

ABSTRACT

A substrate is placed on a support surface of a table. The support surface has a flatness of 2 μm or less. The table has a plurality of holes in the surface. A vacuum is created in the holes to bring the substrate into close contact with the support surface. This eliminates deformations of the substrate, such as twisting and curving. A coating die is moved above the table to apply a coating material onto the substrate. Consequently, a coating having a constant thickness is formed on the substrate.

This is a national stage application of PCT/JP96/02552, filed Sep. 6,1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for coating asubstrate with a coating material by using a coating die and anapparatus for supplying the coating die with the coating material.Particularly, the present invention relates to a method and apparatusfor applying the coating material with a constant thickness onto a thinsubstrate by using the coating die. Further, the present inventionrelates to an apparatus for supplying the coating material. The presentinvention is applicable to a coating system having the coating die, butnot limited thereto.

2. Description of the Related Art

Both Japanese Patent Laid -Open Publications Tokkaihei 4-61955 andTokkaihei 1-135565 disclose a method for applying a photo-resist onto aglass plate. Typically, this method is referred to as spin-coating. Withthis spin-coating, the glass plate is horizontally supported on arotatable spin chuck. The photo-resist is poured onto an upper centralportion of the glass plate. Then, upon rotation of the spin chuckholding the substrate, the poured photo-resist flows outwardly acrossthe entire surface of the glass plate due to a centrifugal force createdby the rotation.

The spin coating, however, can only retain a small part of the pouredphoto-resist (i.e., only about five percent thereof) on the glass plate,and the majority of the photo-resist (i.e., 95 percent thereof) iswasted without being recycled. This waste makes spin-coatingsignificantly expensive.

Japanese Patent Laid-Open Publication Tokkaisho 56-159646 disclosesanother coating method in which a coating die has a distribution slotfor distributing a coating material therefrom. According to this coatingmethod, simply by moving the coating die along a surface of a glassplate, all the photo-resist distributed from the nozzle will be appliedonto the surface of the glass plate. This is more economical thanspin-coating.

However, most glass plates have three-dimensional deformations (e.g.,twisting and curving) and uneven thicknesses. Likewise, a table forsupporting the glass plate includes such three-dimensional deformations.A film, having a thickness of only 10 μm or less, coated by the coatingdie on the substrate supported by the table will have a striped patternwith thin and thick areas due to gap variations between the coating dieand the substrate.

To overcome this problem, Japanese Patent Laid-Open PublicationTokkaihei 7-328513 discloses a method for controlling a coating die.With this method, during coating, gaps (actual gaps) between a nozzle ofthe coating die and each successive portion of a substrate arepre-measured by a range sensor mounted on the coating die. Using themeasured gap values, the coating die is moved towards and away from thesubstrate in order to keep the actual gap constant.

In this approach, however, two processes are required: one process formeasuring the gaps between the nozzle and the substrate; and anotherprocess for calculating deviations between the successive gaps (i.e.,measurements) and a reference gap predetermined for forming a coatingfilm of specific thickness. Also, these processes must be donesimultaneously during coating. However, the latter calculation processrequires a considerable amount time, which prevents the calculationsfrom matching the movement of the coating die. Therefore, this approachcan only be applied when the coating speed is lower than the calculationspeed, and results in an unacceptable delay in coating.

Japanese Patent Laid-Open Publication Tokkaihei 5-185022 disclosesanother coating method. According to this method, a thickness for eachsuccessive portion of a member to be coated (a metal plate) ispre-measured by a sensor at a measurement station at an upstream side ofa coating station with respect to a traveling direction of the metalplate. Using the measured values, an applicator (e.g., coating die orblade) is moved towards and away from the metal plate. In this method,however, the measurement station is spaced apart from the coatingstation. Therefore, if there exists an error or height difference ofabout several micrometers with respect to a direction perpendicular to amajor surface of the metal plate between a first surface portion forsupporting the metal plate at the measurement station and the secondsurface portion for supporting the same at the coating station, it isimpossible to measure such a height difference. This in turn prohibits acorrection for the height difference between a first surface portion atthe measurement station and a second surface portion at the coatingstation. As a result, a gap between the applicator and the metal plateat the coating station can not be adjusted to a predetermined referencegap even by driving a motor to move the coating device towards and awayfrom the metal plate with an aid of a controller.

A device for supplying the coating die with the coating material hasbeen known. This device has a reservoir for accommodating the coatingmaterial and a feed pipe fluidly connected between the reservoir and thecoating die. The feed pipe is provided with a pump for feeding thecoating material into the coating die and a filter so that the coatingmaterial is supplied from the reservoir to the coating die. However,when using a gear pump and a volute pump as the pump, a number of smallbubbles, each having a diameter of about 0.01 mm, are invariably mixedinto the coating material which is discharged from the pump. The smallbubbles hardly affect the thickness of the coated film when the feedpipe has an inner diameter of 5 mm or more.

If, however, the feed pipe has an upwardly curved or bent portion, thesmall air-bubbles can group together to grow into a relatively largebubble having a diameter of 1 mm or more. The large bubble tends tocontract and expand while traveling in the pipe due to changes inpressure from the pump. Then, if the feed pipe has an inner diameter ofabout 5 mm or less, the resultant contraction and expansion leads apressure variation in the coating material in the feed pipe. This inturn results in unevenness in the resultant coating. Particularly, thispressure variation will be problematic when a finished thickness of thecoating is 10 μm or less and when a thickness variation of ±5% or lessis required for the resultant coating in a wet condition.

SUMMARY OF THE INVENTION

Accordingly, the primary object of the invention is to provide a methodand apparatus capable of applying a coating material with a constantthickness to a substrate having three-dimensional deformations (e.g.,twisting and curving) and an uneven thickness by using coating die.

Another object of the invention is to provide a device for supplying thecoating material by which small bubbles created in the coating materialin a pipe are collected at a certain place and then readily discharged.

A method for coating a substrate using a coating die according to thepresent invention comprises the steps of placing a substrate on asupport surface which has a flatness of 2 μm or less and has a pluralityof holes, introducing a vacuum into the holes to draw the substrate intoclose contact with the support surface, and moving the coating dierelative to the substrate and applying to the substrate a coatingmaterial discharged vertically and downwardly from the coating die.

Preferably, the method further comprises the steps of determining acapillary number based upon a viscosity and a surface tension of thecoating material and a moving speed of the coating die relative to thesubstrate, determining a corresponding non-dimensional minimum coatingthickness based upon a relationship between the capillary number and anon-dimensional minimum coating thickness, and determining a gap betweenthe coating die and the substrate according to the correspondingnon-dimensional minimum coating thickness. Preferably, the capillarynumber is 0.1 or less.

An apparatus for coating a substrate using a coating die according tothe present invention comprises a support surface having a flatness of 2μm or less and a plurality of holes defined therein, a mechanism forintroducing a vacuum into the holes to draw substrate placed on thesupport surface into close contact with the support surface, a coatingdie having a slot nozzle directed vertically and downwardly toward thesubstrate for discharging the coating material, a mechanism for movingthe coating die relative to the support surface, a mechanism forsupplying the coating material with the coating die, and a mechanism forcontrolling a gap between the coating die and the substrate.

Another method for coating a substrate using a coating die according tothe present invention comprises the steps of pre-measuring actual gapsin an entire coating area between successive portions of the surface ofthe substrate to be coated and the coating die spaced apart from thesubstrate, calculating deviations between the pre-measured actual gapsand a reference gap, and then moving the coating die for applying thecoating material while the actual gap between the coating die and thesubstrate is adjusted.

Further, another apparatus for coating a substrate using a coating dieaccording to the present invention comprises a support surface forsupporting the substrate horizontally, a coating die having a slotnozzle directed vertically and downwardly toward the substrate fordischarging a coating material, a mechanism for measuring an actual gapin an entire coating area between the substrate supported on the supportsurface and the coating die, a mechanism for calculating a deviationbetween the actual gap and a predetermined reference gap, a mechanismfor moving the coating die relative to the support surface, a mechanismfor supplying the coating material to the coating die, and a mechanismfor adjusting the actual gap between the coating die and the substratebased upon the deviation at coating.

Preferably, the support surface has a flatness of 2 μm or less and aplurality of holes defined therein, and the apparatus further comprisesa mechanism for introducing a vacuum into the holes to draw thesubstrate into close contact with the support surface.

In another aspect of the present invention, an apparatus for supplying acoating material to a coating die in which a tank for accommodating thecoating material is connected through a pipe with the coating die, thepipe having a pump for feeding the coating material from the tank to thecoating die comprises a deaerator which is arranged in the pipe forcollecting bubbles in the coating material and discharging the same froman openable exit (exhaust) defined in an upper portion thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings throughout which like parts are designed by like referencenumerals, and in which:

FIG. 1 shows a sectional view of a coating system of the invention andan electric circuit thereof;

FIG. 2 is a graph showing a relationship between a capillary number anda non-dimensional minimum coating thickness;

FIGS. 3A, 3B, and 3C show a process for applying a coating material ontoa substrate by using a coating die according to the invention;

FIG. 4 shows a relationship between a reference gap, actual gaps, anddeviations;

FIG. 5 shows a general construction of a coating material supply unitused in the coating system shown in FIG. 1;

FIG. 6 is a sectional view of a deaerator employed in the coatingmaterial supply unit;

FIG. 7 shows a modified embodiment of the deaerator;

FIGS. 8A and 8B show arrangements of the deaerator in the coatingmaterial supply unit in which a pump and the deaerator are disposed atthe same level;

FIGS. 9A, 9B, and 9C show other arrangements of the deaerator in thecoating material supply unit in which the coating die is located higherthan the pump; and

FIGS. 10A and 10B show other arrangements of the deaerator in thecoating material supply unit in which the coating die is located lowerthan the pump.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) First Embodiment

FIG. 1 depicts a coating system 10 having a coating die 12 according tothe present invention. The coating system 10 includes the coating die12. The coating die 12 includes therein a manifold 14 extendinglengthwise, one or more through-holes 16 for supplying coating material(for example, photo-resist) into the manifold 14, and a slot nozzle 18formed along the manifold 14 for discharging the photo-resist therefrom.This coating die 12 is arranged so that an outlet 20 of the slot nozzle18 is directed downwards vertically. The through-hole 16 is fluidlyconnected with a coating material supply unit 22 so that thephoto-resist accommodated in the unit 22 is fed via the through hole 16and manifold 14 to the slot nozzle 18, and is finally discharged fromthe outlet 20.

The coating die 12 is secured on a support 24. The support 24 isdrivingly coupled to both a horizontal drive mechanism 26 for moving thecoating die 12 in a horizontal direction indicated by the arrow X whichis perpendicular to the slot nozzle 18 and a vertical drive mechanism 28for moving the same in a vertical direction indicated by the arrow Y.These mechanisms 26 and 28 are electronically connected to a calculationmechanism 30 so that they are independently energized in accordance withoutputs from the calculation mechanism 30. The vertical drive mechanism28 comprises a servo motor and a mechanism having a lead screw and anassociated screw member engaged therewith through a backlash-less thread(threads having less backlash). The backlash-less threads enable thevertical drive mechanism 28 to move the coating die 12 up and down withgreat precision in the order of microns. Further, the coating die 12carries a non-contact range sensor 32 for measuring a gap therefrom toan object to be placed therebelow. The measurement of the sensor 32 isfed into the calculation mechanism 30.

A table is positioned horizontally below the coating die 12 forsupporting a glass plate 50 to be coated. A support surface 36 of thetable 34 is machined to have a flatness of 2 μm or less. The supportsurface 36 includes therein a plurality of grooves 38 arranged in alattice, and the table 34 includes a number of through-holes 40extending from the groves 38 to a bottom surface 42 thereof. The bottomsurface 42 of the table 34 is covered with a hood 44 so that the hood 44encloses therein the holes 40. The hood 44 is fluidly connected to avacuum pump 48 via an exhaust tube 46. Therefore, placing the glassplate 50 on the surface 36 and then energizing the vacuum pump 48 willintroduce a vacuum into the hood 44, through-holes 40 and groves 38.This vacuum draws the glass plate 50 into close contact with the table34, and this close contact eliminates deformations in the glass plate 50such as twisting and curving.

The calculation mechanism 30 includes an actual gap calculating unit 52,a deviation calculating unit 54 and memory unit 56. The actual gapcalculating unit 52 determines, from the measurement of the range sensor32, a gap (actual gap) between the outlet 20 and an underlying referencesurface. The reference surface is at a position where an upper surfaceof an ideal glass plate having no deformations such as curving orunevenness in thickness would be positioned. The deviation calculatingunit 54 calculates a deviation between the actual gap and the referencegap. The reference gap is determined in a manner described below. Thememory unit 56 stores the calculated values as required. Also, thecalculation mechanism 30 vertically controls the coating die 12 bydriving the vertical drive mechanism 28 so that the deviation iseliminated in order to adjust the actual gap to the reference gap.

Incidentally, the reference surface of the ideal glass plate is animaginary surface and therefore the reference gap cannot be measured.For this reason, practically the actual gap (i.e., pseudo actual-gap) isdetermined by measuring a gap between the support surface 36 and theoutlet 20 and then subtracting a thickness of the ideal glass plate 50from the previously measured gap.

In operation, the glass plate 50 is positioned on the table 34. Thisplate 50 is sized so that it entirely covers the grooves 38 formed inthe support surface 36 of the table 34. Then the vacuum pump 48 isenergized. This introduces a vacuum into the hood 44 and the vacuumdraws the glass plate 50 into close contact with the support surface 36of the table 34. Thus, the deformations of the glass plate 50 such astwisting and curving are eliminated. Next, the range sensor 32 measuresthe gap between the support surface 36 and the outlet 20. Then theactual gap between the outlet 20 and the reference surface iscalculated. The calculation mechanism 30 determines a deviation betweenthe actual gap and the reference gap (determination of the reference gapwill be discussed below). Also, the calculation mechanism 30 activatesthe vertical drive mechanism 28 to adjust the actual gap between theoutlet 20 and the glass plate 50 to the reference gap. Subsequently,while maintaining the reference gap, the photo-resist 58 is supplied tothe manifold 14 from the coating supply unit 22 and the coating die 12is moved by the horizontal drive mechanism 26 in the direction indicatedby an arrow X (perpendicular to the slot nozzle 18). The glass plate 50is applied with the photo-resist 58 discharged from the outlet 20through the slot nozzle 18.

The reference gap will be discussed below. The reference gap isdetermined by using a relationship (see FIG. 2) between a capillarynumber obtained from an experiment performed by Lee et al. and anon-dimensional minimum coating thickness, and following equations (1)and (2). The experiment made by Lee et al. is fully described inChemical Engineering Science, vol. 47, No. 7, pages 1, 703 to 1, 713 in1992. The inventors, Yokoyama et al., of the present invention, verifiedthe experimental results by numerical analysis. Consequently, theresults of the analysis were found to be almost identical to those ofthe experiments, as shown in FIG. 2.

In a graph shown in FIG. 2, the capillary number Ca is substantiallyproportional to the non-dimensional minimum coating thickness t (a ratiobetween the capillary number and the non-dimensional coating thickness)if the capillary number Ca is less than a critical capillary number of0.1. Therefore, in the condition that the capillary number Ca is withinthis proportional area, a coating having a thickness which correspondsto the reference gap can be obtained by adjusting the reference gap to acertain value which exists between maximum and minimum gaps.

The maximum gap is calculated by calculating the capillary numberobtained by substituting values representing properties of the coatingmaterial and coating speed (moving speed) into the equation (1), thendetermining a corresponding non-dimensional minimum coating thicknessfrom the above calculated capillary number with reference to the graphin FIG. 2, and finally substituting the above determined correspondingnon-dimensional minimum coating thickness into the equation (2). Theminimum gap, on the other hand, is calculated by determining thecorresponding non-dimensional minimum coating thickness from thethreshold capillary number 0.1 with reference to FIG. 2, and thensubstituting the above determined corresponding non-dimensional minimumcoating thickness into the equation (2). $\begin{matrix}{{Ca} = {\mu \cdot {U/\sigma}}} & (1) \\{{t = {h/H}}{wherein}{{{Ca}\text{:}\quad {Capillary}\quad {number}},{\mu \text{:}\quad {Viscosity}\quad {of}\quad {coating}\quad {material}\quad \left( {{Pa} \cdot S} \right)},{U\text{:}\quad {Coating}\quad {speed}\quad {\left( {m\text{/}s} \right).\sigma}\text{:}\quad {Surface}\quad {tension}\quad \left( {N\text{/}m} \right)},{t\text{:}\quad {Non}\text{-}{dimensional}\quad {minimum}\quad {coating}\quad {thickness}},{h\text{:}\quad {Minimum}\quad {thickness}\quad {of}\quad {coating}\quad {material}\quad \left( {{target}\quad \quad {coating}\quad {thickness}} \right)\quad ({µm})},\quad {and}}{H\text{:}\quad {Gap}\quad {from}\quad {outlet}\quad {to}\quad {plate}\quad {({µm}).}}} & (2)\end{matrix}$

Note that the maximum and minimum gaps are an upper limit gap and alower limit gap, respectively, at which a stable and continuous meniscusof the coating material between the outlet and the plate can be kept.Therefore, if the gap is larger than the maximum gap or smaller than theminimum gap, no stable meniscus will be formed. On the other hand, ifthe gap ranges between the maximum and minimum gaps, the surface tensionof the coating material in the gap serves as a shock absorber. Thesurface tension compensates for the variations of the gap caused by theunevenness of the plate, and thereby ensures the coating die forms acoating with a desired (target) thickness on the plate.

The determination of the maximum and the minimum gaps will be discussedbelow. Assume that the photo-resist 58 has a viscosity (μ) of 0.06 Pa·s,a surface tension (σ) of 30×10⁻³ N/m, and the glass plate 50 is 650 mmlong, 550 mm wide, and 1.1 mm±10 μm thick. Also, it is assumed that acoating speed is 10 mm/s and a target thickness of the coating is 10 μm.In this case, by substituting the viscosity (μ) of 0.06 Pa·S of thephoto-resist 58, coating speed (U) of 10 mm/s, and surface tension (σ)of 30×10⁻³ N/m into the equation (1), the capillary number (Ca) of 0.02is determined. Then, according to the graph in FIG. 2, by using thecalculated capillary number (i.e., 0.02), the correspondingnon-dimensional minimum coating thickness (t) of 0.15 is determined.Finally, by substituting the corresponding non-dimensional minimumcoating thickness t of 0.15 and the target coating thickness of 10 μminto the equation (2), the maximum gap of 66 μm is determined.

The reference gap will be minimized if the capillary number (Ca) isapproximately at the critical capillary number of 0.1. Therefore, fromthe graph in FIG. 2, the corresponding non-dimensional minimum coatingthickness corresponding to the capillary number of 0.1 is determined tobe about 0.6. Then, by using the target coating thickness of 10 μm, theminimum gap of about 16 μm is determined by the equation (2).

Consequently, if the gap from the outlet 20 to the glass plate 50 rangesbetween the minimum gap (i.e., 16 μm) and the maximum gap (i.e., 66 μm),the coating die 12 will form a coating with a thickness of 10 μm on theglass plate 50 in spite of the gap variation due to the unevenness ofthe glass plate 50.

The curving of the plate 50 is substantially eliminated by drawing itonto the support surface 36 of the table 34. The plate 50, however,still includes the unevenness in thickness of ±10.0 μm. Therefore, theupper surface of the plate 50 possibly includes a height error of ±11.0μm against the reference surface (the table 34 has a flatness of 2 μm orless).

However, the gap between the outlet 20 of the coating die 12 and theglass plate 50 can be adjusted from 16 μm to 66 μm. Therefore, in casethat the actual gap is set to 55 μm, even if the gap variation isincreased to the maximum and thereby the outlet 20 takes a position thatis farthest from the glass plate 50, the actual gap is still equal to orless than the maximum value of 66 μm (=55 μm) +11 μm). Even when the gapvariation is decreased to the minimum and thereby the outlet 20 takes aposition closest to the glass plate, the actual gap is 44 μm (=55 μm−11μm). Also if the gap is 44 μm, there still exists a clearance of 34 μm(=44 μm−10 μm) between the surface of the coating having a thickness of10 μm and the outlet 20. This clearance ensures that a sufficient spaceis interposed between the outlet 20 of the coating die 12 and the glassplate 50. Further, increasing the reference gap will minimize theadverse effect due to the unevenness in the surface of the substrate.

It should be understood that the range sensor 32 is not limited to thenon-contact type sensor and a contact-type sensor can also be utilizedinstead. Also, although in the previous vacuum mechanism the hood 44covers the entire bottom surface 42 of the table 34, each of the throughholes 40 in the table 34 may be connected directly through an associatedbranch tube with the exhaust tube 46 for drawing the glass plate 50 intoclose contact with the table 34.

As described above, according to this embodiment, the glass plate issupported on the table having a flatness of 2.0 μm or less and it isdrawn closely to the table, which eliminates the twisting and curving ofthe glass plate. Eventually, the gap between the outlet of the coatingdie and the upper surface of the substrate includes a small errorconsisting exclusively of the unevenness in the thickness of the glassplate and the unevenness of the table (i.e., 2.0 μm or less). This smallerror is canceled by the surface tension of the meniscus formed betweenthe outlet of the coating die and the glass plate. This ensures that afilm applied to the glass plate will have a certain thickness.

Further, by setting the capillary number to 0.1 or less, a coatinghaving a desired thickness can be applied onto the glass plate while thegap from the coating die to the glass plate is as large as possible.

(2) Second Embodiment

Although the gap between the outlet of the coating die to the glassplate is fixed from the beginning to the completion of the coating diein the previous embodiment, the gap may be changed according to theunevenness of the glass plate so as to adjust the gap between the outletand the glass plate to the reference gap.

This application method will be discussed with reference to FIGS. 3A to3C and FIG. 4. In this method, the vacuum pump 48 is energized to drawthe glass plate 50 located on the table 34 to the support surface 36 ofthe table 34, and thereby eliminates the deformations such as twistingand curving. Next as shown in FIG. 3A, the gap from the outlet 20 to theglass plate 50 is adjusted to a specified reference gap Gt (level) thatis at a specific value within a range from 16 to 66 μm. Then as shown inFIG. 3B, the non-contact range sensor 32 is moved by the horizontaldrive mechanism 26 to pre-scan the entire surface of the glass plate 50and is used to determine the actual gaps Ga at certain intervals alongthe glass plate 50. Also, based upon the values measured by the sensor32, the actual gap calculating unit 52 in the calculating mechanism 30determines the actual gaps Ga (i), Ga (i+1), Ga (i+2), . . . between theoutlet 20 and each portion of the plate (these values area not pseudoactual gaps). The actual gap is measured against the glass plate 50.Further, the deviation calculating unit 54 calculates deviations D (i),D (i+1), D (i+2), . . . between the reference gap Gt and thecorresponding actual gaps Ga (i), Ga (i+1), Ga (i+2), . . . . Thecalculated deviations are stored in the memory unit 56. In thiscalculation, consideration is made to a relationship between the coatingspeed (i.e., moving speed of the coating die) and the vertical speed ofthe coating die 12 (i.e., a time delay of the vertical movement withrespect to the horizontal movement). Also, the measurement of the actualgap is performed against the glass plate 50 onto which the coatingmaterial is applied, and therefore it is not necessary to compensate forthe differences between the levels of the measuring position and thecoating position, which would otherwise occur if the two positions arespace apart from each other.

After the calculations have been completed, as shown in FIG. 3C, thecoating die 12 is moved horizontally and the photo-resist 58 is fed fromthe coating material supply unit 22 into the coating die 12. At thisstage, as described before, the coating die 12 is moved up and down bythe vertical drive mechanism 28 according to the deviations D calculatedby taking both the coating and elevation speeds into account. Thisensures that the actual gap from the outlet 20 to the surface of glassplate 50 is always kept constant and thereby a coating having a constantthickness is applied onto the glass plate 50.

As described, prior to the coating, the deviations D between thereference gap Gt and the actual gaps Ga have been calculated. Also, thedeviations are determined by taking the elevation (vertical) and moving(horizontal) speeds of the coating die 12 into account. Therefore, theentire surface of the glass plate 50 can readily be applied with thecoating that has a constant thickness. If the actual gap Ga is measuredby the non-contact range sensor 32, the speed of the coating die 13 maybe lower during measurements than that of the coating speed (duringcoating) to increase the precision of the measurements.

According to the coating method using coating die, the calculation iscarried out in the calculation mechanism 30 by taking the coating speedand the elevation speed of the outlet 20 into account, and therefore theactual gap between the surface of the glass plate 50 and the outlet 20of the coating die 12 can always be kept at the reference gap withoutdepending upon the coating speed. Incidentally, when the coating isaccelerated due to an increase in productivity, a range between themaximum and minimum reference gaps will become narrower. In the coatingmethod in the first embodiment in which the gap from the coating die tothe table is fixed, a large deviation between the actual gap and thereference gap may result. This makes it difficult to form a coating witha constant thickness. In the method according to this embodiment, theactual gap can always be adjusted to the reference gap. According tothis method, because the relationship between the elevation speed of thecoating die 12 (outlet 20) and the coating speed is considered (i.e.,time delay), the coating speed can be changed freely.

Also, this invention may be used with a rigid plate rather than aflexible one. In this case, it is not necessary to draw the plate ontothe table 34.

Further, although the described coating die 12 has only one verticaldriving mechanism 28 and one range sensor 32, it may have two of each.In this case, the vertical driving mechanisms 28 are arranged atopposite sides (ends) of the coating die 12, and the range sensors 32are positioned adjacent to the opposite sides (ends) of the coating die12 so that each of which can cooperate with the associated the verticaldriving mechanism 28. With two pairs of range sensors 32 and thevertical associated driving mechanisms 28, the thickness variations ofthe glass plate on opposite sides can be considered during applicationof the coating.

Furthermore, it is not necessary to calculate the reference gap betweenthe coating die and the glass plate using the capillary number Ca asdescribed in the previous embodiment. Alternatively, the reference gapmay be determined through experimentation (regardless of the capillarynumber). Even in this case, the glass plate should be drawn onto thetable to eliminate the twisting and curving of the glass plate.

As can be seen from the above description, it is not necessary tosimultaneously carry out the measurement, calculation and elevation ofthe outlet, and therefore the coating die can be moved up and down tokeep the reference gap even if the coating speed is relatively high.This ensures that the gap between the plate surface and the outlet is atthe reference gap regardless of the coating speed, even if the substratehas deformations (for example, twisting, curving and unevenness inthickness). As a result, even when the coating has a relatively thinthickness of 10 μm or less and extremely precise coating is required inwhich the accuracy of the thickness should be less than ±5%, the desiredcoating can still be formed.

Further, the non-contact type range sensor is integrally mounted on thecoating die. This permits the measurement to be made at the coatingposition, and allows for a precise measurement of the actual gap.

Furthermore, the flexible plate can be drawn onto the flat table by thesuction mechanism in order to remove its twisting and curving. Thisensures that the coating is applied to a desired thickness.

(3) Coating Material Supply Unit

As shown in FIG. 5, the coating material supply unit 22 has a tank 62for accommodating the coating material. The tank 62 is connected to thecoating die 12 by a pipe 68 having a first pipe 64 and a second pipe 66.The first and second pipes 64 and 66 are connected to each other througha deaerator 70. The first pipe 64 has a pump 72 and a filter 74. Thepump 72 is positioned on a side adjacent to the tank 62, and the filter74 is positioned on the other side thereof. Further, the second pipe 66has a valve 76 adjacent to the coating die 12.

As shown in FIG. 6, the deaerator 70 is generally in the form of cone,and has at its top portion an outlet 78 for discharging air. The outlet78 is connected through a valve 80 to the atmosphere. The valve 80 maybe controlled manually or electrically. To decrease an adverse effect ofpressure variation in the deaerator 70 the deaerator 70 has asignificantly larger volume than a large air bubble that can be formedtherein. The pressure variation is caused by contractions and expansionsof the large air bubble 86 collected adjacent to the outlet 78. Thedeaerator 70 is designed to have a volume that can receive a largeamount of coating material as compared to the volume of the large bubble86 possibly formed therein. In addition, the first pipe 64 extends intothe interior of the deaerator 70 and then terminates near the top ofdeaerator 70. On the other hand, the second pipe 66 is connected to thebottom of deaerator 70. This pipe arrangement ensures that small bubbles82 can be collected in the outlet 78 and further no coating materialwith the small bubbles is discharges into the second pipe 66.

Preferably, where the pipe 68 between the pump 72 and the coating die 12includes a bent portion and extends upward from the pump, or where thepipe 68 connected with the pump 72 extends horizontally and then curvesupward, the deaerator 70 is positioned at or adjacent to a next bentportion which extends in the horizontal or in a downward direction.However, if the pipe 68 extends horizontally from the pump and thencurves downward, the deaerator 70 is preferably positioned at oradjacent to the bent portion. This is because the bubbles tend tocollect in the vicinity of the bent portions.

The pump 72 and the coating die 12 can be arranged in three ways. Thepump 72 may be positioned at the same level as the coating die 12.Further, it may be positioned higher or lower than the coating die 12.

If the pump 72 and the coating die 12 are arranged at the same level asshown in FIG. 8A, in which the pump 72 is connected to the coating die12 through a straight pipe 68 extending horizontally, the deaerator 70may be disposed at any position along the pipe 68. If the pump 72 andthe coating die 12 are connected through the pipe 68 that includes aplurality of bent portions (to avoid obstacles as shown in FIG. 8B), thedeaerator 70 is arranged at or adjacent to a first bent portion 84 wherea first vertical pipe portion adjacent the pump 72 is connected at itsuppermost portion with a subsequent horizontal pipe portion. Likewise,if the coating die 12 is located higher than the pump 72, as shown inFIGS. 9A to 9C, the deaerator 70 is preferably arranged at or adjacentto the bent portion 84.

As shown in FIG. 10A, if the coating die 12 is located at a positionlower than the pump 72, the deaerator 70 is preferably arranged at thefirst bent portion 84 where the pipe 68 extends horizontally then turnsdownward. If, however, the pipe 68 includes an upwardly bent portion toavoid obstacles as shown in FIG. 10B, the deaerator 70 is preferablyarranged at or adjacent to the first bent portion 84 where the pipe 68upwardly extends and then horizontally extends.

During operation of the coating material supply unit 22 for the coatingdie 12 thus constructed, the valve 76 is opened and the other valve 80is closed, and then the pump 72 is energized. The first and second pipes64 and 66, and the deaerator 70 are filled with the coating material toprepare for the coating. At this stage, the valve 80 is opened for awhile to exhaust the air collected in the outlet 78 in the deaerator 70.

Subsequently, the pump 72 feeds the coating material continuously intothe coating die 12. The coating material, flowing from the pump 72through the filter 74 into the deaerator 70, can contain small bubbles82 as shown in FIG. 6. When the small bubbles 82 enter the deaerator 70,the small bubbles 82 move upward due to their buoyancy and grow into thelarge bubble 86 at the outlet 78 as shown in FIG. 6. The large bubble 86is forced out through the outlet 78 due to the pressure of the coatingmaterial during periodic openings of the valve 80. The coating materialfrom which the most bubbles 82 have been removed is then fed into thesecond pipe 66 by the pressure applied from the pump 72. At this time,the volume of the deaerator 70 is relatively large. This decreases thecontractions and expansions of the coating material is fed from thefirst pipe 64. This ensures that the coating material is fed out at aconstant pressure. Then, the coating material is supplied to the coatingdie 12, and it is applied onto the glass plate 50.

Although the substrate is a plate, as in the previous embodiments, itmay be a continuous strip-like member. Also, the deaerator 70 is notlimited to the conical configuration, and the deaerator 70 may haveother configurations that include a small area portion at its topadjacent to the outlet 78 to collect small bubbles 82 in the coatingmaterial.

Further, any pipe arrangement for the deaerator 70 can be utilized whichenables the small bubbles 82 to be collected at the deaerator 70. Forexample, as shown in FIG. 7, the first and second pipes 64 and 66 may beconnected with an upper side portion and lower side portion of thedeaerator 70, respectively.

As described above, the bubbles in the coating material are collected bythe deaerator and then removed out of the coating material supplycircuit. Also, the volume of the collected air is small enough ascompared with that of the deaerator so that no substantial pressurevibration is generated in the coating material regardless of thepressure generated for supplying the coating material. This ensures thecoating material to be fed into the manifold in the coating die is at aconstant pressure. The constant pressure allows the plate to be coatedwith a coating that has constant thickness.

While there is shown and described herein certain specific structuresembodying the invention, it will be known to those skilled in the artthat various modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlying inventiveconcept. Further, the invention is not limited to the particular formsherein shown and described except insofar as indicated by the scope ofthe appended claims.

What is claimed is:
 1. A method of coating, comprising: placing asubstrate onto a support surface having a flatness of 2 μm or less, andwherein said support surface has a plurality of holes; introducing avacuum into said holes to attract said substrate into close contact withsaid support surface in order to eliminate deformations of saidsubstrate; determining a capillary number as a function of a viscosityof a coating material, a surface tension of said coating material, and acoating speed of a coating die relative to said substrate by theequation: Ca=μU/s, wherein Ca=said capillary number, μ=said viscosity(Pa·S), U=said coating speed (m/s), and s=said surface tension (N/m),and wherein said determining said capillary number includes selectingsaid capillary number to be 0.1 or less; determining a correspondingnon-dimensional minimum coating thickness as a function of saidcapillary number; determining a reference gap between said coating dieand said substrate as a function of said corresponding non-dimensionalcoating thickness and a target thickness of said coating material by theequation: H=h/t, wherein H=said reference gap, h=said target thicknessof said coating material, and t=said corresponding non-dimensionalcoating thickness; moving horizontally said coating die relative to saidsubstrate; maintaining said reference gap between said coating die and areference surface of said substrate; and applying said coating materialonto said substrate by downwardly discharging said coating material fromsaid coating die.
 2. A method of coating, comprising: placing asubstrate onto a support surface having a flatness of 2 μm or less, andwherein said support surface has a plurality of holes; introducing avacuum into said holes to attract said substrate into close contact withsaid support surface in order to eliminate deformations of saidsubstrate; determining a capillary number as a function of a viscosityof a coating material, a surface tension of said coating material, and acoating speed of a coating die relative to said substrate by theequation: Ca=μU/s, wherein Ca=said capillary number, μ=said viscosity(Pa·S), U=said coating speed (m/s), and s=said surface tension (N/m),and wherein determining said capillary number includes selecting saidcapillary number to be 0.1 or less; determining a correspondingnon-dimensional minimum coating thickness as a function of saidcapillary number; determining a reference gap between said coating dieand said substrate as a function of said corresponding non-dimensionalcoating thickness and a target thickness of said coating material by theequation: H=h/t, wherein H=said reference gap, h=said target thicknessof said coating material, and t=said corresponding non-dimensionalcoating thickness; pre-measuring actual gaps, over an entire coatingarea of said substrate between successive portions of a surface of saidsubstrate and a coating die spaced apart from said substrate after saidintroducing said vacuum into said holes prior to a coating of saidsubstrate; calculating deviations between each of said pre-measuredactual gaps and a reference gap; moving horizontally said coating dierelative to said substrate after said calculating operation; adjustingan actual gap between said coating die and said surface of saidsubstrate to said reference gap by vertically moving said coating diebased upon said calculated deviations; and applying said coatingmaterial onto said substrate surface of said substrate, as pre-measured,during said horizontal moving and adjusting operations.